Thinking is the porduct of interactions between the neurons.
They communicate through electric and chemical signals. The chemical signals consist of small molecules produced inside the neuron. This production is regulated by the nuclear DNA which is triggered the RNA. The cytoplasmatic RNA is activated by extracellular factors. So in a way environmental factors may influence the amount or when this RNA starts working.

Energy is consumed by the brain cells to allow all this exchange if information between the neurons. This is why when we are hungry it affects our ability to think.

Complexity of thought relates to the number of connections between the neurons. The more connections, the more thiking pathways. If you stop using some skills, like for example statistics, or maths, these connections become weaker and weaker and eventually disapear. But if you get to read the stats book again, it becomes easier to re-learn it than when you did it the first time, becuase the neurons retain the memory of those connections.
To establis a connection between two neurons, the neuronal cell has to grow a dendrite until it meets the other cell. This requires more portein production and more DNA and RNA activity to produce these proteins.